This invention concerns an injection sub for use with dual conduit drill pipe systems and more particularly a field adjustable air diffusion sub having improved injection characteristics.
In dual conduit drill pipe systems the individual segments of the drill pipe string include an inner conduit which is disposed concentrically within an outer conduit. When the segments are joined together, two isolated passageways are defined which extend from one end of the string to the other: the interior of the inner conduit (the central passageway) and the space between the inner and outer conduits (the annular passageway). Air lift techniques may be advantageously used with such dual conduit pipe by pumping drilling fluid and compressed air down the annular passageway. The cuttings from the drill bit and drilling fluid then pass through the drill bit and up to the surface through the inner conduit. It has been found that compressed air injected into the inner conduit assists in lifting the cuttings to the surface.
As disclosed in U.S. Pat. No. 3,978,923 issued to George A. Ford, injection subs have been used to diffuse air from the annular passageway into the inner conduit at preselected points along the drill pipe string. Injection subs such as that disclosed by Ford, while improving the lift characteristics of the dual conduit system, have suffered from certain limitations. Such injection subs introduced air into the inner conduit through a circular array of apertures. Such a circular array has a maximum number of apertures limited to the number that can be placed along one circumference of the inner passageway. It has been found that the use of a large number of small injection apertures reduces the turbulence induced in the inner conduit and results in a more efficient air lift system. Prior art injector subs were limited to a single linear array of apertures and consequently could not realize the full lift potential of the injector sub. Secondly, such prior art injection subs were not field adjustable. The rate of air injection was determined by the size and spacing of apertures formed in one of the major structural elements of the injector sub, and the injection rate could not be altered without replacing this entire element. Thirdly, these injection subs were subject to erosion damage to the major structural elements of the sub. During drill bit operation the drilling fluid contained in the inner passageway is an abrasive slurry of suspended cuttings. Eddies are produced in this abrasive slurry by the injection of air into the inner passageway, and these eddies result in increased erosion of the walls of the inner passageway adjacent to the injection apertures. In the case of prior art injection subs, the injection apertures were formed in major structural elements, and erosion damage near the apertures could require the replacement of the entire element.